1. Field of the Invention
This invention relates to a toothed belt, for example, for driving the overhead cam shaft of an automobile, and more particularly it relates to a toothed belt having great durability that makes possible a long life-span under severe conditions.
2. Description of the Prior Art
Unlike flat belts or V-shaped belts, toothed belts do not slip, and thus have a high transmission efficiency. In addition, compared to transmission apparatus such as gears, chains, etc., the toothed belts have the advantage of very little noise. Thus, toothed belts are used for simultaneous drive in such places as automotive overhead cam shafts, and their use is increasing.
A conventional toothed belt is constructed, as shown in FIG. 1, by a back surface section 1 and tooth sections 2, both of which are made of an elastic substance such as rubber, a tooth cloth 3, which covers the tooth sections 2 and the tooth lands 5 positioned between the adjacent tooth sections 2 to prevent abrasion arising from friction between the belt and the pulleys, and a plurality of core fibers 4 placed on the inner surface of the back surface section 1 in the long direction so as to touch the tooth cloth 3. This toothed belt is used, for example, for simultaneous drive of the overhead cam shaft of an automobile. A toothed belt for the driving of an overhead cam shaft moves at high speeds under a heavy load and at high temperatures. When a large number of pulleys are used, the belt moves with many bends. Because a toothed belt for driving an overhead cam shaft is used under conditions of high temperatures, the elastic substance such as rubber, etc., that is used for the construction of the back surface section 1 and the tooth sections 2 undergoes aging, which gives rise to severe cracking. Because of movement at high speed under a heavy load, the friction between the pulleys and the tooth cloth 3 that touches the said pulleys is great, which easily gives rise to cracking, shear, etc., resulting in damage. Moreover, a high pressure arises in the tooth sections 2 engaged with the pulleys, which easily gives rise to cracking, shear, etc., also resulting in damage. Moreover, when the toothed belt moves with a number of bends due to many pulleys, heat is generated inside of the belt by friction between the core fibers 4 and the elastic substance forming both the back surface section 1 and the tooth sections 2, which causes the acceleration of deterioration of either core fibers or elastic substance. For these reasons, conventional toothed belts for driving overhead cam shafts cannot be operated for long periods of time.
Rubber such as chloroprene rubber (CR) or the like has been used for the back surface section 1 and the tooth sections 2 of conventional toothed belts, but it is inferior in durability. To improve durability when used under severe conditions, kinds of rubber with superior resistance to heat are used, such as epichlorohydrin rubber (CHR), chlorosulphonated polyethylene methylene linkage (CSM), ethylene-propylene-diene methylene linkage (EPDM), etc. However, CHR is inferior in thermosetting and cold resistance. CSM allows the generation of much internal heat and is inferior in cold resistance and oil resistance. EPDM has unsatisfactory oil resistance.
The tooth cloth 3, which functions to prevent abrasion by the friction of the pulleys with the elastic substance of the back surface section 1 and the tooth sections 2, has been made of nylon raw fibers for clothing, which are fine, with the fineness of 2 to 3 deniers for single fibers, and with the relative viscosity with respect to sulfuric acid of around 2.5, so that movement under severe conditions, i.e., at high temperatures, high speeds, and heavy loads, results in serious abrasion. If the tooth cloth is damaged by abrasion, there is a danger that the rubber of the tooth sections 2 and/or the core fibers 4 inside of the tooth lands 5 come into contact with the pulleys, so that the tooth sections 2, etc., cannot be protected, and the toothed belt loses durability.
The core fibers 4 are usually made of twisted bundles of glass fiber filaments. Each of the core fibers 4 takes on some of the load on the toothed belt while the belt moves, and the core fibers also act to prevent stretching of the belt. However, the core fibers 4 cause the generation of internal heat by their friction with the back surface section 1 and/or the tooth sections 2. Internal heat causes accelerated aging of the elastic substance of the back surface section 1 and the tooth sections. Moreover, it causes a decrease in the bonding strength between the elastic substance and the tooth cloth 3.